A transmission electron microscope (TEM) is an instrument for obtaining a TEM image of a specimen by directing an electron beam at the specimen and magnifying and projecting an electron image transmitted through the specimen, for example, onto a fluorescent screen. The thickness of the specimen observed with this transmission electron microscope is required to be less than 200 nm so that the electron beam can penetrate through the specimen. This specimen has been prepared by the prior art procedure described below.
It is assumed that an integrated circuit has been fabricated on a silicon wafer W. For example, when a cross section of this wafer taken in the direction (indicated by t) of the thickness is observed with a transmission electron microscope, the wafer is processed by the following sequence of steps:
(1) As shown in FIG. 25, a plurality of specimens S1 are cut out from the wafer W. PA1 (2) As shown in FIGS. 26, (A) and (B), the extracted specimens S1 are stacked on top of each other in the direction of thickness indicated by t and glued together. PA1 (3) As shown in FIGS. 26, (C) and (D), the glued specimens are cut into a cylinder having a diameter of about 3 mm. PA1 (4) As shown in FIG. 26(E), the cylindrical specimen is sectioned into disks, each about 0.2 mm thick. PA1 (5) Each specimen disk approximately 0.2 mm thick is thinned to a thickness of about 70 .mu.m by a polishing machine. FIG. 26(F) shows a specimen polished to a thickness of approximately 70 .mu.m. PA1 (6) As shown in FIGS. 27, (A) and (B), the specimen with a dimpled center is polished while rotated. As a result, the thickness of the specimen taken in the center is about 10 .mu.m. PA1 (7) As shown in FIG. 27(C), an ion beam is made to impinge on the specimen at a shallow angle to polish the specimen surface with ions. Thus, the thickness of the specimen in the center is less than 200 nm. During observation of the specimen, an electron beam is directed at the center of the specimen thinned in this way.
The prior art specimen preparation work as described above needs sophisticated art. It takes a long time even for a skilled person to prepare the specimen by this technique. Furthermore, it is difficult to accurately bring a desired portion of the wafer W into the center of the specimen S and to thin it.